Theoretical Investigations of CO<sub>2</sub> and H<sub>2</sub> Sorption in an Interpenetrated Square-Pillared Metal–Organic
Material

Simulations of CO<sub>2</sub> and
H<sub>2</sub> sorption and separation
were performed in [Cu­(dpa)<sub>2</sub>SiF<sub>6</sub>-i], a metal–organic
material (MOM) consisting of an interpenetrated square grid of Cu<sup>2+</sup> ions coordinated to 4,4′-dipyridylacetylene (dpa)
rings and pillars of SiF<sub>6</sub><sup>2–</sup> ions. This
class of water stable MOMs shows great promise in practical gas sorption/separation
with especially high selectivity for CO<sub>2</sub> and variable selectivity
for other energy related gases. Simulated CO<sub>2</sub> sorption
isotherms and isosteric heats of adsorption, <i>Q</i><sub>st</sub>, at ambient temperatures were in excellent agreement with
the experimental measurements at all pressures considered. Further,
it was observed that the <i>Q</i><sub>st</sub> for CO<sub>2</sub> increases as a function of uptake in [Cu­(dpa)<sub>2</sub>SiF<sub>6</sub>-i]. This suggests that nascently sorbed CO<sub>2</sub> molecules within a channel contribute to a more energetically favorable
site for additional CO<sub>2</sub> molecules, i.e., in stark contrast
to typical behavior, sorbate intermolecular interactions enhance sorption
energetics with increased loading. The simulated structure at CO<sub>2</sub> saturation shows a loading with tight packing of 8 CO<sub>2</sub> molecules per unit cell. The CO<sub>2</sub> molecules can
be seen alternating between a vertical and horizontal alignment within
a channel, with each CO<sub>2</sub> molecule coordinating to an equatorial
fluorine MOM atom. Calculated H<sub>2</sub> sorption isotherms and <i>Q</i><sub>st</sub> values were also in good agreement with the
experimental measurements in [Cu­(dpa)<sub>2</sub>SiF<sub>6</sub>-i].
H<sub>2</sub> saturation corresponds to 10 H<sub>2</sub> molecules
per unit cell for the studied structure. Moreover, there were two
observed binding sites for hydrogen sorption in [Cu­(dpa)<sub>2</sub>SiF<sub>6</sub>-i]. Simulations of a 30:70 CO<sub>2</sub>/H<sub>2</sub> mixture, typical of syngas, in [Cu­(dpa)<sub>2</sub>SiF<sub>6</sub>-i] showed that the MOM exhibited a high uptake and selectivity for
CO<sub>2</sub>. In addition, it was observed that the presence of
H<sub>2</sub>O had a negligible effect on the CO<sub>2</sub> uptake
and selectivity in [Cu­(dpa)<sub>2</sub>SiF<sub>6</sub>-i], as simulations
of a mixture containing CO<sub>2</sub>, H<sub>2</sub>, and small amounts
of CO, N<sub>2</sub>, and H<sub>2</sub>O produced comparable results
to the binary mixture simulations.